This patent specification relates generally to oil and gas field applications. More particularly, this patent specification relates to devices and methods for detecting mercury in hydrocarbon fluids and/or natural gas during oil and gas field applications.
The are many approaches for the development of sensors for detecting concentrations of selected components in gaseous mixtures, for example determining mercury and hydrogen sulfide concentrations are cold vapour atomic fluorescence spectroscopy (CV-AFS) and cold vapour atomic absorption spectroscopy (CV-AAS), which, although extremely sensitive, have certain serious limitations. These methods are used in laboratories and could hardly be used at the well site or even in a downhole application. There are other methods to determine the mercury and hydrogen sulfide concentrations like X-ray fluorescence, neutron activity analysis, atomic emission spectroscopy and mass spectroscopy. However, they all are suitable for laboratory applications and lack suitability for oilfield applications such as well site and/or downhole applications.
Typically, mercury is trapped on gold, silver or an activated charcoal for determining concentration and sampling purposes. In the oil and gas industry, the sampling uses of gold amalgamation and further analysis are done in a remote laboratory away from the well site and/or downhole.
Some of the known approaches for the development of a mercury sensor include the use of thin film gas sensors developed to detect a selected component in a composite gas. For example, a thin film gas sensor is formed of a suitable semiconductor material whose electrical resistivity changes in response to the adsorption of the selected component. The thin film gas sensor can include a gold thin metal film layer deposited on a substrate, wherein the resistivity of the gold changes in response to the adsorption of mercury. The electrical resistance of the gold film exposed to the gas is then measured and can provide a basis for determining the concentration of the selected component.
The adsorption of mercury to gold as a surface process can result in the diffusion of the gold being much slower than the adsorption on the surface and uptake from the surface. As long as the amount of mercury adsorbed is lower than the maximum surface concentration value, the adsorption occurs with a sticking probability close to unity. The slower diffusion leads to saturation of the gold surface with mercury and block further adsorption. This effect can occur when about 50% of the gold surface is saturated with mercury. An increasing temperature decreases the amount of absorbed mercury.
Further, an increased mercury exposure time up to hours and/or very high mercury concentrations leads to the formation of mercury aggregates in the form of islands or three-dimensional dendritic structures on/in the gold layer. This is considered one of the main limitations in the implementation of thin film gold sensors for mercury monitoring.
Another disadvantage of gold layer mercury sensors is their poor selectivity. The sensor has a cross-sensitivity towards water vapour, sulphuric acid vapour, sulfides, thiols and iodine. It was shown that the use of self-assembled monolayers of hexadecanethiol can decrease the sensitivity to these components dramatically except for iodine. Mercury has been shown to be able to penetrate the monolayer and give a response that is close to 50% of a bare electrode.
A number of transducers have been used for the detection of mercury based on its absorbance to gold. Most of them use a gold layer with a particular thickness. Transducers that measure the increase in mass can include quartz microbalance, surface acoustic wave and micro-cantilevers. The transducer principles that use optical techniques can include surface Plasmon resonance whereas localized plasmon resonance is suggested but not implemented for mercury sensors. Finally, the adsorption of mercury by gold leads to an increase in surface electrical resistance. Conductometric transducers can measure particular levels of mercury concentrations. The sensitivity of this type of measurement decreases with an increase of the thickness of the gold layer. Desorption of the adsorbed mercury can be achieved by heating the sensor to very high temperatures; however, it comes with at a cost.
For example, adsorptive thin film gas sensors can be regenerated after adsorbing a sufficient amount of the selected component to trigger an indication circuit. The regeneration of the thin film involves heating the thin film to a very high temperature to liberate the molecules of the selected component adsorbed by the thin film layer, i.e., the gold film, to prepare the gas sensor for a new cycle of gas detection and measurement. Depending upon the type of molecules adsorbed, the regeneration temperature can be a very high temperature. In prior art devices, the thin film layer is commonly used in both the sensing role and as a heater conductor for regeneration.
However, thin film layers that are commonly used as both the sensing role and as a heater conductor for regeneration which results in a limited operational life. One of the main reasons for the failure of the mechanism can involve the electromigration of the gold metal in the sensing film. Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in microelectronics and related structures. As the structure size in electronics such as integrated circuits (ICs) decreases, the practical significance of this effect increases. Thus, the result of electromigration is that metal atoms move from the thin gold film into the dividing layers on a chip. If electromigration occurs to a great degree, and enough metal atoms move into the dividing layers, the thin gold film may become too thin, resulting in failure of the gas sensor. So, electromigration can be furthered when the thin film layer is used as a sensor and as a heater conductor. The sensor and heater thin film gas sensor likely fails after a small number of cycles of sensing and regeneration due to the high re-evaporation temperature.
Along with the problem of electromigration, the sensor and heater thin film gas sensors have a lower sensitivity that is needed. The sensitivity of a combined sensor and heater thin film gas sensor can be dictated by its design. Another problem with the sensor and heater thin film gas sensors is that the resistance of their trace can be high. Therefore, a high voltage (approximately 60-100 volts) is needed to regenerate the sensor. Consequently, the sensor and heater thin film gas sensors are often limited in use to areas where 120 VAC or suitable power generators are available.
Some prior art thin film gas sensors circumvent the above-noted problems found with the sensor and heater thin film gas sensors by utilizing external heating elements to heat the thin metal film to the regeneration temperature. Unfortunately, such external heating elements can be difficult to manufacture and to calibrate for specific sensor applications. Moreover, the amount of heat generated by such a heating element may vary over the surface of the sensing layer. Uneven heating is undesirable because it can cause insufficient or inconsistent regeneration.
Therefore, there is a need for methods and systems for detecting mercury and hydrogen sulfide in hydrocarbon fluids and natural gas during oil and gas field applications.